1. Field of the Invention
The present invention relates to a plastic molding apparatus and a plastic molding method for semiconductor devices and, more particularly, to a plastic molding apparatus and a plastic molding method for semiconductor devices in which plastic molding is performed by using a transfer molding press.
2. Description of the Related Art
Plastic molding apparatus for semiconductor devices are generally designed to perform molding by feeding a resin tablet in a transfer pot into cavities under pressure by means of a plunger after upper and lower die plates mounted on a transfer molding press have been clamped.
The aforementioned type of conventional plastic molding apparatus for semiconductor devices includes: an upper die plate, having a transfer pot in which a plunger for pressurizing a resin tablet is advanced or retracted, and cavities which define one half of a plurality of resin packages; a lower die plate disposed below the upper die plate in such a manner that it can advance or retract, with cavities defining the other halves of the packages; and an ejector mechanism accommodated in each of the upper and lower die plates and having ejector pins which move into and away from the cavities.
Since the transfer pot is open to the atmosphere in the thus-arranged plastic molding apparatus for semiconductor devices, the air entering the transfer pot during insertion of the plunger is mixed into the molten resin due to the pressurization of the plunger and forms voids in the packages which seal the semiconductor chips.
Hence, in the aforementioned type of plastic molding apparatus for semiconductor devices, a suction device has been used during resin molding to evacuate the interior of the transfer pot and cavities and thereby prevent entry of air into the packages.
FIG. 9 is a schematic view of the conventional plastic molding apparatus for encapsulating semiconductor devices. In the figure, a lower chase block 1 and an upper chase block 2, each having cavities (not shown), are respectively fixed to a lower surface table 3, in which a heater (not shown) is incorporated, and an upper surface table 4. A lower die 5 includes the lower chase block 1, the lower surface table 3 and a lower ejector mechanism (not shown), and is fixed to a lower platen 7 for pressing. An upper die 6 includes the upper chase block 2, the upper surface table 4 and an upper ejector mechanism (not shown) and is fixed to an upper platen 8 for pressing. An upper die sealing cover 9 is disposed on the outer periphery of the lower die 5, and is fixed to the lower platen 7. An upper die sealing cover 10 is disposed on the outer periphery of the upper die 6, and is fixed to the upper platen 8. A mating surface between the lower die sealing cover 9 and the upper die sealing cover 10 is sealed by means of a sealing material 11. The upper die sealing cover 10 is provided with an evacuation duct 13 connected to an evacuation device (not shown) so as to evacuate the interior of the lower die sealing cover 9 and upper die sealing cover 10, thereby forming a vacuum chamber 12.
In the thus-arranged conventional plastic molding apparatus for semiconductor devices, the plastic packaging operation is performed in the manner described below. First, semiconductor devices die bonded and wire bonded to leadframes are mounted on the lower chase block 1. Next, clamping is performed by driving a press mechanism to obtain the configuration shown in FIG. 9. After clamping, a resin tablet is charged into a chamber (not shown) in the upper die 6, a plunger (not shown) is fitted into the chamber, and an evacuation device is activated to evacuate the entire interior of the apparatus. When a predetermined degree of vacuum is obtained, the plunger is activated to press the resin tablet and thereby inject the resin, whereby packages sealing the semiconductor devices are molded.
A similar conventional plastic packaging apparatus for semiconuctor devices to that described above is disclosed in Japanese Patent Laid-Open No. 63-64331.
The aforementioned type of plastic molding apparatus has a large overall size. Also, since the volume of the vacuum chamber 12 is large, a long evacuation time and a large evacuating device are necessary.
Furthermore, a change in the package types requires removal of the lower die sealing cover 9 and the upper die sealing cover 10 and loading and unloading of the upper die 6 which weighs about 1000 Kg. Thus, it takes about two days including the time for heating to complete an upper die exchange.